The present invention relates to a unique seal structure for improving axial sealing of secondary air flow in the wheel spaces of gas turbines.
Obtaining high performance levels in a gas turbine requires minimizing leakages of secondary air throughout the wheel spaces. This presents a challenge since the sealing mechanism must be devised to provide a means to effectively seal between rotating components (buckets/blades/disks/spacers) and stationary components (nozzles/vanes/diaphragms). It is common practice to use labyrinth type seals which restrict the area where the leakage might occur and also create a series of pressure loss mechanisms to further reduce the flow of air leakage. Different arrangements of labyrinth seal teeth have been used, some aligned circumferentially, some staggered circumferentially. Also, different numbers of seal teeth are commonly used in series to provide additional pressure losses and further reduce leakage when needed.
The labyrinth seal teeth can be designed to interfere with and cut into the opposing wall, which is usually honeycomb or an alternative abradable material, to provide a minimal gap and leakage area during operation. However, most large gas turbines experience additional closure during hot start-up transients which results in the seal teeth cutting deeper into the abradable wall during the transient start but then opening to expose an enlarged gap during steady state operation.
Another method to seal between the rotating and stationary components used along with labyrinth seals is to install brush seals in series. Brush seals can further reduce leakages, but they are costly and increase the complexity of the gas turbine. Also, there is a limited length that the brush seal bristles can be extended beyond the housing that contains them, and if the transient closure is too large, brush seals cannot be used without risk of a hard rub between the brush seal housing and the rotating components.